Membrane process for separating sulfur compounds from FCC light naphtha

ABSTRACT

A process for the separation of sulfur compounds from a hydrocarbon mixture using a membrane is provided. Preferred hydrocarbon mixtures are oil refining fractions such as light cracked naphtha. Membranes are composed of either ionic or non-ionic materials and preferentially permeate sulfur compounds over other hydrocarbons. A single or multi-stage membrane system separates the hydrocarbon mixture into a sulfur-rich fraction and a sulfur-lean fraction. The sulfur-lean fraction may be used in fuel mixtures and the sulfur-rich fraction may be further treated for sulfur reduction.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Provisional U.S. Application60/258,583 filed Dec. 28, 2000.

BACKGROUND OF THE DISCLOSURE

[0002] 1. Field of the Invention

[0003] The present invention relates to a process for the separation ofsulfur compounds from hydrocarbon mixtures using a membrane.

[0004] 2. Background of the Invention

[0005] Sulfur compounds are impurities in gasoline that compromisevehicle emission controls by poisoning the catalytic converter. In aneffort to further decrease emissions, the U.S. government has recentlyproposed a nationwide reduction of sulfur in gasoline from currentlevels at 300-1000 ppm to an average of 30 ppm (Federal Register,64(92), May 13, 1999). Gasoline producers, both domestic and foreign,selling fuel in the U.S. would be expected to comply by the year 2004.

[0006] Presently, the conventional process for reducing sulfur contentin gasoline involves hydrotreating in which sulfur compounds areconverted to volatile hydrogen sulfide and other organics. This energyintensive process, requiring elevated temperature and pressure, isexpensive for obtaining the proposed lowered sulfur levels. Alternativeprocesses with more efficient sulfur-reducing technology are needed tomaintain progress toward cleaner burning fuels.

[0007] The use of membrane separation technology, in which selectcompounds or types or compounds can be separated from an organic mixtureby permeation through a membrane, has been reasonably well developed.Separation processes that incorporate membranes present an attractiveoption for large-scale purification of petroleum fractions because oftheir inherent simplicity, versatility, and low energy consumption.

[0008] Typically, membrane separation processes rely on the affinity ofa specific compound or class of compounds for the membrane. In this way,the components of a mixture with specific affinity for the membrane willselectively sorb onto the membrane. The sorbed compounds diffuse, orpermeate, through the membrane and are removed on the opposite side.Continual withdrawal of permeated compounds from the membrane maintainsthe driving force for the separation process. Removal of permeatedcompounds is usually achieved by pervaporation or perstraction methods.Pervaporation employs a vacuum on the permeate side of the membrane,removing the permeated compounds in gaseous form, while perstractionemploys a liquid sweep stream, continually washing away permeate.

[0009] The chemical properties of the membrane dictate the type ofcompound that has affinity for it. Some types of membranes are composedof charged chemical groups and are, therefore, considered ionic incharacter. An example of an ionic membrane is Nafion® (available fromDuPont, of Wilmington, Del.), which is a polymer of perfluorosulfonicacid that has been used principally in the dehydration of liquid organicmixtures as described in U.S. Pat. No. 4,846,977. Only few examplesexist for the use of Nafion® in separating organic compounds. U.S. Pat.No. 4,798,764 describes the separation of methanol from dimethylcarbonate or methyl t-butyl ether. The use of Nafion® membranes for theseparation of mixtures of styrene and ethylbenzene has also beenreported (Cabasso, Ind. Eng Chem. Prod Res. Dev. 1983, 22, 313). U.S.Pat. No. 5,498,823 reports the enhanced separation of unsaturatedorganic compounds using silver ion-exchanged Nafion® membranes. Arelated ionic membrane composed of sulfonated polysulfone has been alsoused for the separation of aromatics and non-aromatics as disclosed inU.S. Pat. No. 5,055,631. To date, the use of ionic membranes, such asNafion®, for the separation of sulfur compounds from liquid organicmixtures has not been reported.

[0010] In contrast to ionic membranes, non-ionic membranes are made fromthose materials lacking charged chemical groups. Chemical affinity forthese membranes is usually governed by the hydrophilic or hydrophobicnature of the membrane material. Hydrophilic membranes have affinity forwater or other polar compounds, and those membranes with affinity forwater are often water-soluble. Hydrophilic membranes include both ionicand non-ionic membranes. However, the non-ionic membranes generallycontain polar chemical groups such as hydroxyl, carboxyl, sulfonyl,carbonyl, or amine groups. Examples of hydrophilic non-ionic membranesinclude polyvinylalcohol (PVA), cellulose acetates, and polyvinylamine.Hydrophobic membranes, on the other hand, have little affinity for wateror polar compounds and generally lack or contain a small proportion ofcharged or polar chemical groups. Examples of hydrophobic membranesinclude polyethylene and polystyrene.

[0011] A wide variety of non-ionic membranes have been used inseparation processes. U.S. Pat. Nos. 5,905,182, 5,019,666, 4,997,906,4,944,880, 4,532,029, 4,802,987, 4,962,271, 5,288,712, 5,635,055,3,556,991, 3,043,891, and 2,947,687 describe the separation of aromaticsfrom hydrocarbon mixtures using a wide variety of non-ionic membranematerials. Non-ionic membranes have also been used in the separation ofaromatics containing heteroatoms from hydrocarbon mixtures as disclosedin U.S. Pat. Nos. 5,643,442 and 5,396,019. The aforementioned patents,which are incorporated herein by reference, disclose membrane separationprocesses directed to the separation of aromatics and non-aromaticsusing hydrophobic membranes.

[0012] The proposed mandate for lowered sulfur levels in gasoline hasmade it imperative to improve or replace existing methods fordesulfurization of petroleum fractions. A more cost-effective method forreducing sulfur content in petroleum fractions is a primary goal of theoil refining industry. Current membrane separation technology showspotential for meeting future standards, but has not yet been usedspecifically for this purpose.

SUMMARY OF THE INVENTION

[0013] This invention relates to a process for the separation of sulfurcompounds from hydrocarbon mixtures, preferably oil refining fractions,using a membrane. The membrane may be composed of any material, ionic ornon-ionic, that preferentially permeates sulfur compounds overhydrocarbons. The hydrocarbon mixture is split by one or more membranesforming sulfur-rich and sulfur-lean fractions. The sulfur-lean fractionmay be incorporated into fuel mixtures and the sulfur-rich fraction mayundergo further treatment for reduction of sulfur levels.

[0014] The present invention provides a process for separating sulfurcompounds from a hydrocarbon mixture containing at least one sulfurcompound and hydrocarbons comprising the steps of:

[0015] (a) contacting said hydrocarbon mixture with a first compartmentof a membrane module, said membrane module further comprising a secondcompartment and a hydrophilic membrane separating said first compartmentand said second compartment;

[0016] (b) selectively permeating said sulfur compounds of saidhydrocarbon mixture through said membrane such that a sulfur-richfraction accumulates in said second compartment and a sulfur-leanfraction is retained in said first compartment; and

[0017] (c) retrieving said sulfur-rich fraction from said secondcompartment and said sulfur-lean fraction from said first compartment.

[0018] The present invention also provides a process which furthercomprises the steps of:

[0019] (d) contacting said sulfur-rich fraction of step (c) with a firstcompartment of a further membrane module, said further membrane modulecomprising a second compartment and a hydrophilic membrane separatingsaid first compartment and said second compartment;

[0020] (e) selectively permeating sulfur compounds of said sulfur-richfraction of step (d) through said membrane such that a furthersulfur-rich fraction accumulates in said second compartment and afurther sulfur-lean fraction is retained in said first compartment; and

[0021] (f) retrieving said further sulfur-rich fraction and said furthersulfur-lean fraction;

[0022] (g) repeating steps (d), (e) and (f) using said sulfur-richfraction until a final sulfur-rich fraction of desired sulfur content isobtained; and

[0023] (h) retrieving said final sulfur-lean fraction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a schematic showing a process for the separation ofsulfur compounds from light cracked naphtha using a membrane system.

[0025]FIG. 2 is a schematic showing a process for the separation ofsulfur compounds from light cracked naphtha using a multi-stagedmembrane system under pervaporative conditions.

[0026]FIG. 3 is a diagram of a spiral wound membrane module.

DETAILED DESCRIPTION OF THE INVENTION

[0027] As used herein, “hydrocarbon mixtures” means both syntheticmixtures and oil refining fractions, each of which contain sulfurcompounds. Preferred hydrocarbon mixtures include FCC gasoline mixturesand light cracked naphthas (LCN). Hydrocarbons in the mixture encompassaliphatic, aromatic, saturated, and unsaturated compounds composedessentially of carbon and hydrogen. Preferred hydrocarbons are compoundsthat are commonly found in oil refining fractions including, but notlimited to, benzene, toluene, napthenes, olefins and parrafins. Thesulfur compounds in the hydrocarbon mixtures may be in anyconcentration, but levels of from about 1 ppm to about 10,000 ppm arepreferred, and levels of from about 10 ppm to about 4000 ppm are morepreferred. Also, the term “sulfur compounds” means inorganic or organiccompounds comprising at least one sulfur atom. Preferably, sulfurcompounds of the present invention are thiophenes and derivativesthereof.

[0028] As used herein, “permeate” refers to the portion of thehydrocarbon mixture that diffuses across a membrane, and “retentate”refers to the portion of the hydrocarbon mixture that does not passthrough the membrane. Accordingly, the term “permeate side” refers tothat side of the membrane on which permeate collects which is also thesecond compartment of the membrane module. The term “retentate side”refers to that side of the membrane which contacts the hydrocarbonmixture and also refers to the first compartment of the membrane module.In addition, the term “sulfur-rich” means having an increased content ofsulfur relative to the hydrocarbon mixture and “sulfur-lean” meanshaving a decreased content of sulfur relative to the hydrocarbonmixture.

[0029] According to the present invention, the term “perstraction”refers to a method for removing permeate from a membrane moduleinvolving a liquid sweep stream. In perstraction, a liquid sweep streamis passed through the second compartment of the membrane module, orpermeate side of the membrane, preferably countercurrent to thedirection of flow of the hydrocarbon mixture on the retentate side. Thepermeate dissolves into the sweep stream and is carried away by theflow, thereby preventing the accumulation of preferentially permeatedcomponents such as sulfur compounds. The sweep liquid preferably hasaffinity for, and is miscible with, the permeated components. Methanolis a preferred sweep liquid for membrane systems employing Nafion®-typemembranes as it would also serve as a transport agent for enhancing fluxand selectivity of the membrane.

[0030] As used herein, the term “pervaporation” refers to another methodof removing permeate from the membrane module. In this method, thepermeate is removed from the permeate side of the membrane as a vapor.Thus, a vacuum or lowered pressure must be maintained on the permeateside such that permeated components of the mixture will vaporize upontransfer across the membrane. Transfer of the permeable componentsacross the membrane is ultimately driven by the difference in vaporpressure between the liquid hydrocarbon mixture on the retentate side ofthe membrane and the partial pressure of the permeate vapor on thepermeate side of the membrane.

[0031] As used herein, “hydrophilic” means having an affinity for wateror polar compounds. Additionally, “ionic” means having acidic or chargedchemical groups and “non-ionic” means having neutral chemical groups.

[0032] According to the present invention, “membrane system” is acomponent of a process that preferentially separates sulfur compoundsfrom hydrocarbon mixtures. The membrane system is single-stagedcomprising one membrane module, or multi-staged, comprising more thanone membrane module. “Membrane module” refers to a membrane assemblycomprising a membrane, feed and permeate spacers, and support material,assembled such the membrane separates a first compartment from a secondcompartment. The membrane module may be formed in any workableconfiguration such as flat sheet, hollow fibers, or spiral-wrapped.

[0033] As used herein, “transport agent” refers to an additive in thehydrocarbon mixture for augmenting flux and selectivity of theseparating membrane. Transport agents include, but are not limited to,alcohols, glycols, ethers or any other compounds that are miscible withhydrocarbon mixtures, are sorbed by the ionic membrane, and increaseflux through the membrane. Preferred transport agents are alcohols. Alow boiling transport agent such as methanol is a more preferredtransport agent because of ease of removal by distillation. The quantityof transport agent added to the hydrocarbon mixture is preferably about1% to about 20% by weight. Addition of about 10% by weight of methanolis more preferred. The transport agent also may comprise the sweepstream in perstraction processes.

[0034] As used herein, “Nafion®-type membrane” refers to a polymer ofperfluorosulfonic acid or a derivative thereof. Derivatives include, butare not limited to, Nafion®-type membranes having undergone ion-exchangeor reaction with organic bases. “Nafion,” according to L. Gardner'sChemical Synonyms and Tradenames, 9^(th) ed., 1989, is defined as aperfluorosulphonic acid membrane (DuPont).

[0035] The hydrocarbon mixtures treated by the present inventionencompass both synthetic mixtures and authentic oil refining fractions,each of which contain sulfur compounds. Preferable hydrocarbon mixturesinclude FCC gasoline mixtures and light cracked naphthas (LCN). Thesulfur compounds in the hydrocarbon mixtures may be in anyconcentration, but levels of from about 1 ppm to about 10,000 ppm arepreferred, and levels of from about 10 ppm to about 4000 ppm are morepreferred. Sulfur compounds include organic and inorganic compounds.Preferred sulfur compounds are organic compounds. More preferred sulfurcompounds are thiophenes and derivatives thereof. Hydrocarbons in themixture include, but are not limited to, aliphatic, aromatic, saturatedand unsaturated compounds composed essentially of carbon and hydrogen.Preferred hydrocarbons are compounds that are commonly found in oilrefining fractions including, but not limited to, benzene, toluene,naphthenes, olefins and paraffins.

[0036] A transport agent may be optionally added to the hydrocarbonmixture to augment flux and selectivity of the separating membrane.Preferred transport agents include, but are not limited to, alcohols,glycols, ethers or any other compounds that are miscible withhydrocarbon mixtures and enhance flux through a membrane. More preferredtransport agents are alcohols. A low boiling transport agent such asmethanol is even more preferred. The quantity of transport agent addedto the hydrocarbon mixture is preferably about 1% to about 20% byweight. Addition of about 10% by weight of methanol is more preferred.

[0037] According to the present invention, membrane separation of sulfurcompounds from hydrocarbon mixtures involves the selective permeation,or diffusion, of sulfur compounds through a membrane. Generally, selectdiffusion of components of a mixture is controlled by the affinity ofthe components for the membrane. Components having greater affinity forthe membrane permeate more rapidly. Thus, in the present invention,membranes which have affinity for, or preferentially permeate, sulfurcompounds are preferred. Membranes can be of any suitable composition,and incorporate either or both inorganic and organic materials.Membranes may also possess either ionic or non-ionic properties. Ionicmembranes generally contain charged chemical groups including salts andacids, while non-ionic membranes contain neutral chemical groups.

[0038] Preferred ionic membranes according to the present inventioninclude Nafion®-type acidic membranes, such as Nafion® 117, that havebeen optionally treated by ion-exchange reactions or with bases. Nafion®belongs to a class of solid superacids that generally exhibit acidstrength greater than 100% sulfuric acid. Nafion® is stronglyhydrophilic. Nafion® is preferred for selectively permeating sulfurcompounds which are generally more polar than other components ofpetroleum fractions and other hydrocarbon mixtures. Ion-exchangedNafion® membranes, in which the acidic protons are replaced by othercations, are also within the scope of this invention. Examples ofsuitable cations include, but are not limited to, inorganic ions such assilver, copper, sodium, and organic ions such as tetraalkylammoniums andtetraalkylphosphoniums. In another aspect of the present invention, theNafion®-type membranes may be treated with organic bases including, butnot limited to, triethanolamine and pyridine, thereby forming organicsalts. Nafion®-type membrane modification by reaction with organic basesresults in increased selectivity for sulfur compounds over saturates andolefins.

[0039] Ionic membranes generally perform best in the presence of atransport agent. For example, when a Nafion®-type membrane is contactedwith a transport agent, it swells from sorbtion of the transport agent,changing the microstructure of the polymer such that flux through themembrane is enhanced. Transport agents preferably include alcohols,glycols, ethers or any other compounds that are miscible withhydrocarbon mixtures, are sorbed by the ionic membrane, and increaseflux through the membrane.

[0040] Non-ionic membranes are also suitable for the present invention.Preferred membranes are composed of hydrophilic materials including, butnot limited to, cellulose triacetate (CTA) and polyvinylpyrrolidone(PVP). Hydrophilic properties generally enhance the selective membranepermeation of sulfur compounds in hydrocarbon mixtures. Furthermore, notransport agent is generally required to observe reasonable levels offlux and selectivity when using non-ionic membranes. In fact, the PVPand CTA membranes show a surprising, but desirable, simultaneousincrease in flux and selectivity upon increasing temperature of feed.This result is in contrast to what has been observed for hydrophobicmembranes, such as polyimides, under similar conditions which usuallyshow a decrease in selectivity and an increase in flux with increasingtemperature.

[0041] Other types of membranes include inorganic membranes comprisingceramics, inorganic oxides, metal foils, or carbon.

[0042] The present invention encompasses a process for the separation ofsulfur compounds from hydrocarbon mixtures. According to the process, ahydrocarbon mixture is divided into a sulfur-rich fraction, i.e.,sulfur-rich permeate, and a sulfur-lean fraction, i.e., sulfur-leanretentate, using a membrane system. The sulfur-rich fraction, orsulfur-rich permeate, corresponds to the portion of the hydrocarbonmixture that diffused through the membrane. The sulfur-lean fraction, orsulfur-lean retentate, corresponds to the portion of the hydrocarbonmixture that did not pass through the membrane. The hydrocarbon mixturetreated by the process is preferably light cracked naphtha (LCN);however, any oil refining fraction or organic mixture contaminated withsulfur compounds is suitable. The sulfur compounds in the hydrocarbonmixtures may be in any concentration, but levels of from about 1 ppm toabout 10,000 ppm are preferred, and levels of from about 10 ppm to about4000 ppm are more preferred.

[0043] Incorporated into the membrane separation process is a membranesystem which separates sulfur compounds from hydrocarbon mixtures. Themembrane system can be single-staged comprising one membrane module, ormulti-staged comprising more than one membrane module. Each module hasat least two compartments, a first compartment and a second compartment,separated by a membrane assembly, the assembly preferably comprising amembrane, feed spacers, and support material. The first compartmentreceives the hydrocarbon mixture in liquid form while the secondcompartment collects the portion of the hydrocarbon mixture that haspermeated through the membrane. The permeate is removed from the secondcompartment to maintain a chemical gradient that drives the transfer ofsulfur compounds across the membrane.

[0044] Removal of the permeate is accomplished by either perstraction orpervaporation. In perstraction, a liquid sweep stream is passed throughthe second compartment of the membrane module, preferably countercurrentto the direction of flow of the hydrocarbon mixture in the firstcompartment. The permeate dissolves into the sweep stream and is carriedaway by the flow, thereby preventing the accumulation of preferentiallypermeated components such as sulfur compounds. The sweep liquidpreferably has affinity for, and is miscible with, the permeatedcomponents. Methanol is a preferred sweep liquid for membrane systemsemploying Nafion®-type membranes as it would also serve as a transportagent for enhancing flux and selectivity of the membrane.

[0045] Under pervaporative conditions, the permeate is removed from thesecond compartment as a vapor. Thus a vacuum or lowered pressure must bemaintained in the second compartment such that permeate will vaporizeupon transfer across the membrane. The driving force for transportacross the membrane is the difference in vapor pressure between theliquid hydrocarbon mixture and the permeate partial pressure. Vaporizedpermeate can be subsequently condensed with a chiller. The vapor iscooled and condensed to a liquid and may be optionally heated prior todelivery to subsequent membrane modules. A detailed discussion ofperstraction and pervaporation can be found in Membrane Handbook, W. S.Ho and K. K. Sirkar, Eds., Chapman and Hall, 1992, herein incorporatedby reference.

[0046] According to the present invention, the permeate is enriched insulfur and corresponds to the sulfur-rich fraction. The retentate isdepleted in sulfur and corresponds to the sulfur-lean fraction. In amulti-stage membrane system, the permeate of the initial membrane modulemay be optionally treated by another membrane module, and the permeateof that module further treated by another, proceeding indefinitely untila desired sulfur concentration is obtained in the permeate. Thesulfur-lean retentate, exiting the membrane system preferably containsabout 1 ppm to about 300 ppm sulfur, more preferably about 1 ppm toabout 100 ppm, and most preferably about 1 ppm to about 50 ppm. Thesulfur-lean fraction ideally can be used directly in fuel formulation.Permeate can be combined with other sulfur-containing hydrocarbonmixtures, such as heavy cracked naphtha (HCN), for conventional removalof sulfur compounds by hydrotreating. The hydrotreated stream can beoptionally combined with the sulfur-lean fraction for further refiningor fuel formulation.

[0047] Membrane modules are of reasonable size and shape, includinghollow fibers, stretched flat sheet, or preferably, spiral-woundenvelopes. In the spiral-wound configuration (FIG. 3), the open sides ofmembrane envelopes are positioned and sealed over a permeate receptaclesuch as perforated piping. The envelopes are spirally wrapped around thereceptacle to minimize volume. Feed spacers, such as, for example,plastic netting or nylon mesh, separate the membrane envelopes to allowpenetration of the hydrocarbon mixture between the wrapped layers. Theinterior of each membrane envelope is fitted with a permeate spacer tochannel permeate toward the receptacle. The permeate spacer is composedof a material that is flexible, porous, and inert such as polyester. Themembrane preferably is a composite comprising a stiff but flexibleporous backing which is directed toward the inside of the envelope.Backing materials are preferably resistant to organic mixtures andinclude polyester, ceramic, glass, paper, plastic, or cloth. Cushionscomposed of a flexible, inert material may flank either side of thepermeate spacer inside the membrane envelope and contribute tostructural integrity of the membrane assembly under applied pressure.

[0048] The membrane itself preferably possesses certain qualities tofunction effectively in a process for separating sulfur compounds fromhydrocarbon mixtures. In addition to selectivity for sulfur compounds,desirable membrane qualities include resistance to operative conditionssuch as thermal stress, sustained pressure, and prolonged contact withorganic chemical mixtures. Membrane thickness may vary from about 0.1microns to about 200 microns, but thinner membranes are preferred formaximum flux such as, for example, membranes having a thickness of about0.1 microns to about 50 microns, or more preferably, about 0.1 micronsto about 1 micron.

[0049] Preferred non-ionic membranes of the present invention arefabricated according to a proprietary method developed by MembraneTechnology and Research, Inc. (MTR) of Menlo Park, Calif. MTR membranedesigns are disclosed in U.S. Pat. Nos. 4,931,181; 4,963,165; 4,990,255;and 5,085,776, which are also incorporated herein by reference. Thesemembranes are composite membranes prepared in a two-step process. Thefirst step involves the deposition of a microporous support layer,comprising polysulfones, polyimides, or polyamides, onto a flexibleporous backing made of an inert material (i.e., polyester fabric,ceramic, glass, paper, plastic, or cotton). The second step involvescoating the microporous layer with a dilute solution of polymer,resulting in a thin, defect-free, selectively permeable layer which isresponsible for the selectivity of the membrane.

[0050] Typical process conditions according to the present inventiondepend on several variables including membrane separation method (i.e.,pervaporation vs. perstraction) and feed composition. Determination ofappropriate pervaporative and perstractive operating conditions is wellwithin the capabilities of one skilled in the art. Some typicaloperating parameters for perstractive processes of the present inventioninclude feed flow rates of from about 30 to about 50 gpm, absolutemembrane flux of from about 0.5 to about 150 kg·m⁻²·D⁻¹, feedtemperature of from about 20° C. to about 300° C., and negligiblepressure drop across the membrane. Additionally, some typical operatingparameters for pervaporative processes of the present invention includefeed flow rates of from about 30 to about 50 gpm, absolute membrane fluxof from about 0.5 to about 150 kg·m⁻²·D⁻¹, feed temperature of fromabout 20° C. to about 300° C., and lowered pressure on the permeate sidemeasuring from about 1 to about 80 mmHg.

[0051] Advantages of the present invention are numerous. The separationof sulfur compounds from hydrocarbon mixtures such as oil refiningfractions allows the concentration of sulfur contaminants such that asmaller total volume of liquid needs to be processed by conventionalhydrotreating. Additionally, selectivity of the membrane for sulfurcompounds over unsaturated hydrocarbons results in a low olefin contentin the sulfur-rich stream and reduced octane loss and hydrogenconsumption during the hydrotreating process.

[0052] Those skilled in the art will appreciate that numerous changesand modifications may be made to the preferred embodiments of thepresent invention, and that such changes and modifications may be madewithout departing from the spirit of the invention. It is, therefore,intended that the appended claims cover all such equivalent variationsas fall within the true spirit and scope of the present invention.

EXAMPLES Example 1

[0053] Pervaporative Process for Reducing Sulfur Content in LightCracked Naphtha Using a Membrane System.

[0054] This process is outlined in FIG. 1. Light cracked naphtha (LCN)originating from an FCC main column and containing 1990 ppm of sulfur isfed at a rate of 35 gpm into a membrane system and maintained at apressure of 50 psig. The membrane system is composed of onespiral-wrapped membrane module. The membrane comprises a thin layer ofpolyvinylpyrrolidone (PVP), derived from a 0.5% aqueous PVP solution,layered on porous polyvinylidenefluoride. The permeate side of themembrane module is held under vacuum at a pressure of 50 mm Hg.Sulfur-rich permeate vapor exits the membrane system and is condensedwith a chiller operating at 80° F. The liquid is combined with heavycracked naphtha (HCN) derived from the FCC main column, and the mixtureis processed by conventional hydrotreating. The sulfur-lean retentateexiting the membrane system, containing about 120 ppm sulfur, iscombined with the hydrotreated fraction of similar sulfur content. Thecombined streams are used directly in the gasoline pool, diluted by twovolume equivalents ultimately giving a fuel product with 30 ppm sulfurcontent.

Example 2

[0055] Perstractive Process for Reducing Sulfur Content in Light CrackedNaphtha Using a Membrane System.

[0056] Light cracked naphtha originating from an FCC main column andcontaining 1700 ppm sulfur is fed at a rate of 35 gpm into a membranesystem at ambient pressure. The membrane system is composed of onemembrane module, and the membrane has a thickness of about 50 micronsand comprises Nafion® 117 supported on woven polyester. A sweep streamcomposed of methanol is fed to the permeate side of the membrane at arate of 35 gpm at ambient pressure. The sulfur-rich permeate, containingabout 1 weight % of sulfur, mixes with the methanol sweep stream. Theresulting mixture is fed to a distillation unit in which the methanol isremoved from the sulfur-rich permeate. The distilled sulfur-richpermeate is combined with heavy cracked naphtha (HCN) from the FCC maincolumn and hydrotreated. Similarly, the sulfur-lean retentate,containing about 150 ppm sulfur and 5 weight % methanol, exiting themembrane system, is fed to a separate distillation unit in which themethanol is removed. The resulting methanol-lean retentate fraction isused in gasoline mixtures after combining with hydrotreated HCN.

Example 3

[0057] Multi-Stage Pervaporative Process for Reducing Sulfur Content inLight Cracked Naphtha (LCN) Using a Membrane System.

[0058] This process is outlined in FIG. 2. LCN originating from an FCCmain column containing 1880 ppm sulfur is fed to a membrane system. Themembrane system is composed of two membrane modules, a first stagemembrane module and a second stage membrane module. Both modules areoperated under pervaporative conditions and have a membrane comprisingcellulose triacetate (CTA) mounted on porous polyvinylidenefluoride. LCNinitially enters the first module on the retentate side of the membraneand sulfur-rich permeate vapor, containing about 0.5% by weight sulfur,is drawn out of the permeate side. First stage permeate vapor iscondensed with a chiller and heated to a temperature of 120° C. beforeentering the second stage membrane module on the retentate side of themembrane. As in the first stage module, the permeate from the secondstage module is drawn away as a vapor and condensed. Sulfur content ofthe permeate from the second stage module is enriched to 0.93% byweight. The sulfur-rich permeate from the second stage module is mixedwith HCN (1 weight % sulfur) from the FCC main column and hydrotreated.The hydrotreated mixture, containing about 150 ppm sulfur, is combinedwith retentate from both of the membrane modules for a combined sulfurcontent of about 150 ppm.

What is claimed is:
 1. A process for separating sulfur compounds from ahydrocarbon mixture containing at least one sulfur compound andhydrocarbons comprising the steps of: (a) contacting said hydrocarbonmixture with a first compartment of a membrane module, said membranemodule further comprising a second compartment and a hydrophilicmembrane separating said first compartment and said second compartment;(b) selectively permeating said sulfur compounds of said hydrocarbonmixture through said membrane such that a sulfur-rich fraction collectsin said second compartment and a sulfur-lean fraction is retained insaid first compartment; and (c) retrieving said sulfur-rich fractionfrom said second compartment and said sulfur-lean fraction from saidfirst compartment.
 2. The process of claim 1 further comprising thesteps of: (d) contacting said sulfur-rich fraction of step (c) with afirst compartment of a further membrane module, said further membranemodule comprising a second compartment and a hydrophilic membraneseparating said first compartment and said second compartment; (e)selectively permeating sulfur compounds of said sulfur-rich fraction ofstep (d) through said membrane such that a further sulfur-rich fractionaccumulates in said second compartment and a further sulfur-leanfraction is retained in said first compartment; and (f) retrieving saidfurther sulfur-rich fraction and said further sulfur-lean fraction; (g)repeating steps (d), (e) and (f) using said sulfur-rich fraction until afinal sulfur-rich fraction of desired sulfur content is obtained; and(h) retrieving said final sulfur-lean fraction.
 3. The process of claim1 wherein said hydrocarbon mixture is obtained from an oil refiningprocess.
 4. The process of claim 1 wherein said hydrocarbon mixture is alight cracked naphtha.
 5. The process of claim 1 wherein said sulfurcompound is thiophene or a derivative of thiophene.
 6. The process ofclaim 1 wherein said hydrophilic membrane is an ionic membrane.
 7. Theprocess of claim 6 wherein said membrane is selected from the polymersof perfluorosulfonic acid and derivatives thereof.
 8. The process ofclaim 1 wherein said hydrophilic membrane is a water-soluble membrane.9. The process of claim 1 wherein said hydrophilic membrane is anon-ionic membrane.
 10. The process of claim 10 wherein said membranecomprises polyvinylpyrrolidone.
 11. The process of claim 10 wherein saidmembrane comprises cellulose triacetate.
 12. The process of claim 1which is performed under pervaporation conditions.
 13. The process ofclaim 1 which is performed under perstraction conditions.
 14. Theprocess of claim 2 wherein said membrane of said further membrane moduleis an ionic membrane.
 15. The process of claim 14 wherein said membraneis selected from the polymers of perfluorosulfonic acid and derivativesthereof.
 16. The process of claim 2 wherein said hydrophilic membrane isa non-ionic membrane.
 17. The process of claim 2 wherein saidhydrophilic membrane is a water-soluble membrane.
 18. The process ofclaim 16 wherein said membrane comprises polyvinylpyrrolidone.
 19. Theprocess of claim 16 wherein said membrane comprises cellulosetriacetate.